An experimental study of the forced convection heat loss from long thin cylinders placed normal to an incompressible flow was carried out. Tests were made in three different gases (air, argon, helium) at atmospheric pressure and in air over a pressure range 0.2 to 8 atm. Particular attention was given to determining the effect of cylinder temperature on the heat loss. It was found that the constants employed in the usual empirical representation of this heat loss vary with cylinder temperature in a way dependent on Knudsen number, results in helium giving a marked indication of this. A velocity fluctuation sensitivity equation was derived which includes the effects of this temperature variation.